M4A Converter Mac (http://www.m4aconvertermac.com/) is the best Mac tool to convert iTunes M4A audios.
You can use it to convert iTunes M4A songs to MP3 for playback on Zune, Creative Zen, Archos, iRiver, Walkman, etc.

This powerful tool supports all popular video/audio formats as the input and have profiles optimized for various portable devices. All codecs are built in, you can customize the encoding settings and use the powerful video editing functions.